Contactless quasi-steady-state photoconductance (QSSPC) characterization of metal halide perovskite thin films

We apply the contactless quasi-steady-state photoconductance (QSSPC) method to co-evaporated methyl ammonium lead iodide (MAPbI3) perovskite thin-films. Using an adapted calibration for ultralow photoconductances, we extract the injection-dependent carrier lifetime of the MAPbI3 layer. The lifetime is found to be limited by radiative recombination at the high injection densities applied during the QSSPC measurement, enabling the extraction of the electron and hole mobility sum in the MAPbI3 using the known coefficient of radiative recombination of MAPbI3. Combining the QSSPC measurement with transient photoluminescence measurements, performed at much lower injection densities, we obtain the injection-dependent lifetime curve over several orders of magnitude. From the resulting lifetime curve, we determine the achievable open-circuit voltage of the examined MAPbI3 layer.

A. Coevaporation of MAPbI3 thin film 500 nm thick methylammonium lead iodide (MAPbI3) perovskite layers were deposited on 2.5 x 2.5 cm 2 borofloat glass substrates. Lead iodide (PbI2) and methylammonium iodide (MAI) were thermally co-evaporated in a lab-type evaporation chamber (Mini SPECTROS, Kurt J. Lesker Company) inside a glovebox with nitrogen atmosphere. The borofloat glass substrates were initially cleaned in an ultrasound bath in three consecutive steps using mucasol, isopropanol and acetone. They were then rinsed with water to remove any remaining soap residues.
Subsequently, the substrates were placed in a substrate holder, where the temperature was kept at 25 °C during the entire deposition process. After the chamber pressure was decreased to below 2 x 10 -6 Torr the preheating started. First, the PbI2 crucible was heated to 200°C. Reaching that temperature, the MAI crucible was heated to 100°C while the PbI2 crucible was further heated to 300°C. After opening the crucible shutters and reaching a PbI2 deposition rate of 1.08 Å/s as well as a MAI crucible temperature of 118°C, the co-evaporation process started with the subsequent opening of the substrate shutter. To ensure a laterally homogeneous deposition, the substrate holder was rotated at 20 rpm. Details of our process can be found in Refs. S1 and S2.

B. Time-Resolved Photoluminescence (TRPL)
TRPL measurements were performed using a FluoTime 300 tool from PicoQuant. The measurement is based on a 20 ps (FWHM) short-pulse laser illumination of the sample at 100 kHz with a 505 nm laser. It includes an ultra-precise detection of single photons via a TimeHarp 260 photon counter with a base resolution of 25 ps. By selecting a low excitation energy of 26 pJ/cm 2 we decrease the probability of a second undetectable photon in each 10 µs laser-pulse-cycle to below 1%. Thereby, only one, if any, photon is detected in each cycle. The emission time is tracked and the photon is counted in a histogram which consists of time bins with a width of 800 ps. Over many excitation-cycles, the histogram forms a decay curve. We then interpret the resulting decay curve as discussed in Ref. 3. The dynamic nature of the TRPL does not allow to measure the actual excess carrier lifetime during the excess carrier decay. However, we have estimated the excess carrier concentration to be much lower (≤10 15 cm -3 ) in our MAPbI3 film compared to the QSSPC measurement. Figures S1 shows an exemplary measurement of the measured PL signal PL 1/2 as a function of time after the laser excitation pulse. The lifetime approaches in the asymptotic limit a constant value of τTRPL = 3 µs, which we identify with the Shockley-Read-Hall lifetime τSRH.

Figure S1: Exemplary time-resolved photoluminescence (TRPL) measurement of a 500 nm thick MAPbI3 layer on borofloat glass using a Fluotime 300 spectrometer by PicoQuant. For t > 2 µs the PL decay curve becomes asymptotic and results in a lifetime of τTRPL = 3 µs.
The TRPL measurement depicted in Figure S1 features an initial multiexponential decay. For longer times of t > 2 µs the asymptotic decay can be fitted monoexponentially with a decay time of τSRH = τTRPL = 3 µs which indicates a high electronic thin-film quality.

C. X-Ray Diffraction (XRD)
XRD measurements were performed with a Malvern Panalytical Empyrean XRD tool using CuKα radiation (λ = 1.54 Å). The XRD measurement shown in Figure S2 depicts two clear peaks associated with the pure tetragonal (β) phase of MAPbI3. The larger Peak at 2θ of 14.07° is associated with the MAPbI3 (110) lattice orientation while the smaller peak at 2θ of 28.04° represents the second-order reflex of MAPbI3 at (220) lattice orientation [S3, S4]. Another peak at 2θ of 12.6° indicates the presence of PbI2. Figure S2 clearly shows that the tetragonal MAPbI3 phase is the dominant phase in our films, but PbI2 is also present. Figure S3: Photoluminescence measurement of a 500 nm MAPbI3 thin film on borofloat glass using a Fluotime 300 spectrometer by PicoQuant.

D. Spectral photoluminescence
For spectrally resolved measurements of the photoluminescence (PL) signal of the MAPbI3 layer, we use a Fluotime 300 spectrometer by PicoQuant. Figure S3 shows a spectral measurement of the emitted PL intensity, resulting in a well-shaped PL peak with a peak wavelength of 774 nm, corresponding to a bandgap of 1.6 eV, as typical for high-quality tetragonal MAPbI3 at room temperature [S5,S6].